Mercedes tests solid-state battery in adapted EQS

Mercedes-Benz has started testing a solid-state battery from partner Factorial Energy in a modified EQS this month. In the coming weeks, the Stuttgart-based company wants to collect data on the battery’s efficiency, durability, and performance.

Road tests with the new solid-state battery in a ‘1,000 km’ EQS test car started in February 2025. According to Mercedes, the vehicle had to be slightly adapted to test the technology in the EQS, particularly in the battery installation space and on the battery housing.

The battery pack should offer a range of around 1,000 kilometers. Market maturity is planned “before the end of this decade.” Mercedes promises that the solid-state battery will offer a 25% greater range than a lithium-ion battery of the same size and weight.

Factorial Energy

Development partner and supplier Factorial Energy is a U.S. specialist for solid-state battery cells supported by Mercedes-Benz, Stellantis, and Hyundai-Kia. The company is pursuing several lines of development. In June 2024, Factorial already delivered B samples of its solid-state cells with a charging capacity of more than 106 Ah to Mercedes-Benz. These should achieve an energy density of 391 Wh/kg.

At the same time, the company is working explicitly with Mercedes on cells with the brand name Solstice. So far, they only have A sample status, but they are expected to score points with an even higher energy density and a dry cathode coating process.

In September 2024, Factorial promised an energy density of up to 450Wh/kg for its Solstice product, which would increase the range of electric cars by up to 80%. The company also recently announced the next step: to increase this cell type to a charging capacity of 40 Ah.

“Being the first to successfully integrate lithium metal solid-state batteries into a production vehicle platform marks a historic achievement in electric mobility,” said Siyu Huang, CEO and Co-Founder of Factorial Energy.

“This breakthrough demonstrates that solid-state battery technology has moved beyond the laboratory and into real-world application, setting a new benchmark for the entire automotive industry. Our collaboration with Mercedes-Benz proves that the future of electric vehicles is not just a vision but a reality we’re delivering today,” he added.

Engineers combined

Mercedes-Benz engineers from the road and racetrack and Factorial cell engineers have worked together to deliver an all-new solid-state battery test program, bringing the first car powered by a lithium-metal solid-state battery to the road.

“Mercedes AMG High-Performance Powertrains (HPP), a 100 % subsidiary of Mercedes-Benz Group specializing in industry-leading Formula 1 technology, and the Mercedes‑Benz Center of Competence for Battery Systems designed and developed a completely new and innovative battery system. In addition to its prowess on the racetrack, HPP can rapidly transfer F1 technologies and know-how into high-performance automotive projects,” says the press release.

After intensive testing on various test benches, the prototype solid-state battery was integrated into an EQS at the end of 2024. At the same time, the first laboratory vehicle tests were conducted in Stuttgart to prepare for the road tests that started in February 2025.

“Developing an automotive-scale solid-state battery underlines our commitment to innovation and sustainability,” said Markus Schäfer, Mercedes board member and Chief Technology Officer, Development & Procurement.

“We’re therefore excited to announce that we’ve started road testing with a prototype vehicle equipped with this advanced technology. We will gain crucial insights into possible series integration of this cutting-edge battery technology,” he added.

The technology

Solid-state batteries use a solid electrolyte instead of a liquid one, which enhances cell safety and allows for new anodes like lithium metal. Thus, they significantly outperform conventional lithium-ion cells. In combination with a lithium-metal anode, they also enable next-level energy densities.

Solid-state technology can potentially increase the gravimetric energy density of vehicle batteries up to 450 Wh/kg at the cell level, thereby increasing the driving range. Gravimetric energy density refers to the energy stored in a battery cell per unit mass.

This metric is crucial for evaluating the efficiency and performance of battery cells, especially in applications where weight is a critical factor, such as in electric vehicles. Solid-state battery technology reduces the battery weight while improving cell safety.

The solid-state battery from Mercedes‑Benz features an innovative floating cell carrier, for which a patent has already been granted. When the battery charges, the materials expand, and when it discharges, it contracts. The volume change in solid-state cells refers to the expansion and contraction of the materials inside the battery during charging and discharging.

To support the cells during these volume changes, the Mercedes‑Benz solid-state battery is equipped with pneumatic actuators that interact with the cell volume change during charging and discharging, which affects the battery’s performance and lifespan.